What Do Ventilated Shelf Installation and Measurement Systems Analysis Have in Common?

Minitab Blog Editor 17 April, 2017

Ventilated ShelfHave you ever tried to install ventilated shelving in a closet?  You know: the heavy-duty, white- or gray-colored vinyl-coated wire shelving? The one that allows you to get organized, more efficient with space, and is strong and maintenance-free? Yep, that’s the one. Did I mention this stuff is strong?  As in, really hard to cut? 

It seems like a simple 4-step project. Measure the closet, go the store, buy the shelving, and install when you get home. Simple, right? Yeah, it sounded good in my head!

The lessons I learned in this project underscore the value of doing measurement system analysis in your quality improvement projects, with statistical software such as Minitab. Whatever you're trying to accomplish, if you don't get reliable measurements or data, the task is going to become more challenging.

Before Process Map

Well it turned out to be more complicated and involved a lot of rework. Did I mention that this shelving is made of heavy gauge steel that is nearly impossible to cut with ordinary tools? So, my simple 4-step process turned into a 7-step process with lots of rework (multiple trips to the store to have the shelves re-cut).

My actual process looked more like this!

After Process Map

All the sources of variation from Measurement Systems Analysis (MSA) apply here: Repeatability, Reproducibility, Bias, Linearity, and Stability.  Let’s review these terms and see how I could have done better at measuring the closet, the first time.

Components of Measurement Error

When it was time to measure the closet, I had a few measuring-device choices hanging around my garage: a yardstick, a cloth tape measure, and a steel tape measure. 

Bias examines the difference between the observed average measurement and a reference or master value. It answers the question: "How accurate is my gage when compared to a reference value?" Unless there is visible damage, all three of these measuring devices should be acceptable for my shelf project.

Stability is the change in bias over time. Measurement stability represents the total variation in measurements obtained on the same part measured over time, also known as drift. It is important to assess stability on an ongoing basis. While calibrations and gage studies provide some information about changes in the measurement system, neither provides information on what is happening to the measurement process over time. But unless there is visible damage, all three of these measuring devices should be acceptable for use.

Linearity examines how accurate your measurements are through the expected range of the measurements. It answers the question: "Does my gauge have the same accuracy across all reference values?"  If you use the yardstick or steel tape measure, then the answer might be “yes” because of its solid construction.  But the cloth tape measure could stretch when extended, making it less reliable at longer lengths. Examine the cloth measuring tape for evidence of stretching or wear. If damage is present, do not use the measuring device.

Repeatability represents the variation that occurs when the same appraiser measures the same part with the same device. This is best represented with the advice “Measure twice, cut once!” In my case, if I had measured the closet width multiple times, I would have realized I was getting a different answer each time and therefore needed to take better care when measuring. Then I could have gotten more accurate measurements for each shelf. 

Reproducibility represents the variation that occurs when different appraisers measure the same part with the same device. In my case, if I'd asked my son to measure the same locations that I just measured, I would have discovered that we got different answers: I should have accounted for the mounting brackets in my measurements. (The fact that he did is why he’s in school to become a Mechanical Engineer.)

In summary, my afternoon shelf installation project ended up taking two days to complete, resulting in multiple trips to the store, a lot of frustration for me, and late dinners for my family because I was too busy to cook! 

My lessons learned from this project are:

  1. Don’t assume your closet walls are exactly parallel at the top, middle and bottom of the closet. Instead, measure at each location where a shelf is to be installed.  Remember the Rule of Thumb for Gage R&R: take measurements representing the entire range of process variation.
  2. Apply the Gage R&R sources of measurement error when measuring:
    1. Visually inspect the measuring device before using to verify it is in good condition.
    2. Measure twice, cut once. (Repeatability)
    3. Ask my family for assistance in measuring.  (Reproducibility)
  3. Did you know that you can purchase a laser measure for about $30 these days?  If only I had known…
  4. Consider hiring a professional because this project was harder than it originally seemed.